1. Field of the Invention
The present invention relates to an apparatus and method for requesting packet retransmission in a wireless communication system.
2. Description of the Related Art
The Universal Mobile Telecommunication Service (UMTS) system is a 3rd generation asynchronous mobile communication system that uses Wideband Code Division Multiple Access (WCDMA) based on Global System for Mobile communications (GSM) and General Packet Radio Services (GPRS), both of which are European mobile communication systems.
The 3rd Generation Partnership Project (3GPP), which is now in charge of the UMTS standardization, a discussion is being made on a Long Term Evolution (LTE) system as the next generation mobile communication system of the UMTS system.
The LTE system is a technology for realizing high-speed packet-based communication at about 100 Mbps, aiming at commercialization in around 2010. Regarding the commercialization of the LTE system, a discussion is being held on several schemes: one scheme for reducing the number of nodes located in a communication path by simplifying a configuration of the network, and another scheme for maximally approximating wireless protocols to wireless channels.
The LTE system uses Hybrid Automatic Retransmission reQuest (HARQ) as a scheme for increasing transmission efficiency of high-speed packets. Since it is not possible to meet various Quality-of-Service (QoS) requirements only with HARQ, an upper layer can use Automatic Retransmission reQuest (ARQ) distinguishable from HARQ.
A brief description of HARQ and ARQ will be given below.
HARQ is a technique for soft-combining previously received data with its retransmitted data without discarding the previously received data, thereby increasing a reception success rate. More specifically, a receiving-side HARQ entity determines the presence/absence of an error in a received packet, and sends an ACKnowledged (HARQ ACK) signal or a Non-ACKnowledged (HARQ NACK) signal to a transmitting side according to the presence/absence of an error. The transmitting side then retransmits a corresponding missing HARQ packet or transmits a new HARQ packet according to the HARQ ACK/NACK signal. The HARQ technique is characterized by soft-combining a retransmitted packet with its associated previously received packet to reduce an error occurrence probability.
ARQ is a technique for checking sequence numbers of received packets, and sending a retransmission request for a missing packet (a packet with a missing sequence number). ARQ does not soft-combine a previously received packet with its associated retransmitted packets.
In the existing wireless communication system, since ARQ and HARQ both serve to recover errored packets (packets in which an error has occurred), there is an opinion that there is no need to run the two techniques together. However, since it is hard to obtain a sufficiently low packet error ratio only with HARQ, there is an opinion that most packet services should undergo both ARQ and HARQ.
This is because in the HARQ technique, an HARQ ACK/NACK signal is defined as a 1-bit response signal, making it difficult to satisfy a low error rate through channel coding. When an HARQ NACK signal is misrecognized as an HARQ ACK signal in the wireless situation (‘HARQ NACK/ACK error’), a corresponding HARQ packet is completely lost in an HARQ level (between HARQ layers). The reliability of the HARQ ACK/NACK signal serves as an important factor for the decision on a packet error ratio in the HARQ level. The wireless communication system intends to support fast retransmission of a missing packet by applying HARQ and ARQ.
FIGS. 1A and 1B are diagrams illustrating a structure of a radio layer and a packet structure of the corresponding particular layer in a wireless communication system, respectively.
Referring to FIG. 1A, a radio layer structure (i.e., radio protocol structure) includes upper layers 110 and 115, ARQ layers 120 and 125, a Medium Access Control (MAC) layer 130 and a PHYsical (PHY) layer 140.
The upper layers 110 and 115 are protocol stacks configured separately for each service, and for example, an AMR codec/Real Time Protocol/User Datagram Protocol/Internet Protocol (codec/RTP/UDP/IP) or File Transfer Protocol/Transmission Control Protocol/Internet Protocol (FTP/TCP/IP) layer can be the upper layer.
The ARQ layers 120 and 125 can be configured for each service on a one-to-one basis, and satisfy a required Quality of Service (QoS).
The MAC layer 130 is connected to a plurality of ARQ layers 120 and 125, and multiplexes a plurality of ARQ packets to one HARQ packet. Further, the MAC layer 130 performs an HARQ operation on the multiplexed HARQ packet.
The physical layer 140 performs an operation of transmitting/receiving an HARQ packet over a wireless channel.
Referring to FIG. 1B, an ARQ packet is a packet reconfigured by allocating a sequence number to the data delivered from the upper layer so that the packet can undergo ARQ. An HARQ packet is a unit packet which is transmitted and received on the actual wireless channel through an HARQ operation.
The ARQ packet includes an ARQ packet header 161 composed of Sequence Number (SN) 163, size information 164 and framing information 165, and a payload 162 to which the actual data delivered from the upper layers 110 and 115 is allocated. For example, if an IP packet 150 is transferred from the upper layers 110 and 115 to the ARQ layers 120 and 125, the ARQ layers 120 and 125 can transmit all of the upper-layer IP packet 150 or can transmit only a part of the IP packet 150 taking into account the wireless channel situation or the scheduling situation. This is determined taking the QoS into consideration.
The sequence number 163 is a sequence number sequentially assigned to an ARQ packet 160, and the size information 164 is information indicating a size of the ARQ packet 160. Using the sequence number 163, the ARQ layers 120 and 125 store sequence numbers of ARQ packets in order, or configure ARQ packets by assembling. The framing information 165 is information used for allowing a receiving side to reconfigure a received packet based on a framing operation, and to normally restore it to the original upper layer packet (IP packet). The term ‘framing’ as used herein refers to a series of operations for reconfiguring the IP packet 150 delivered from the upper layers 110 and 115 in an appropriate size.
An HARQ packet 170 is composed of a multiplexing header 171 and a payload. The multiplexing header 171 includes multiplexing information of the ARQ packet 160. For example, an identifier 120/125 of a corresponding ARQ layer among the ARQ layers can be the multiplexing information. The payload is composed of at least lone of the multiplexed ARQ packets. It is obvious that such radio protocol structure and packet structure of a particular layer are commonly applied to the transmitting side and the receiving side.
FIG. 2 is a diagram illustrating an HARQ operation. More specifically, shown in FIG. 2 is a structure for performing HARQ between a transmitting side and a receiving side. When the structure supports an uplink packet service, a terminal serves as a transmitting side and a base station serves as a receiving side. On the contrary, when the structure supports a downlink packet service, a terminal serves as a receiving side and a base station serves as a transmitting side.
Referring to FIG. 2, when the wireless communication environment is considered in which various types of services coexist and can be supported, a transmitting side includes a plurality of upper layer blocks 280 and a multiplexing (MUX) block 275, and a receiving side includes a plurality of upper layer blocks 205 and a demultiplexing (DEMUX) block 210.
The upper layer blocks 205 and 280 can be regarded as, for example, a set of services requiring the same QoS, and for convenience, a flow occurring in one upper layer will be referred to herein as a ‘QoS flow’.
The multiplexing block 275 serves to insert multiplexing information into the data generated by the several upper layers 280 and deliver the data to an HARQ block 272. On the other hand, the demultiplexing block 210 serves to deliver the data provided from an HARQ block 212 to an appropriate upper layer(s) using the multiplexing information included therein. In comparison with FIG. 1A, it can be understood that the multiplexing block 275 and the demultiplexing block 210 are devices included in the ARQ layers.
The HARQ blocks 212 and 272, devices for performing an HARQ operation, are each composed of several HARQ processors. The term ‘HARQ processor’ as used herein refers to a unit device that controls the transmission/reception of an HARQ packet. A transmitting-side HARQ processor is controls the transmission and retransmission of a user packet, and a receiving-side HARQ processor is controls the reception of an HARQ packet and transmission of a response signal. The ‘response signal’ as used herein includes an HARQ ACKnowledgement (HARQ ACK)/HARQ Negative ACKnowledgement (HARQ NACK) signal.
The HARQ blocks 212 and 272 exist in pairs in the transmitting side and the receiving side, and one pair of HARQ blocks 212 and 272 includes a plurality of HARQ processors, thereby enabling continuous transmission/reception of packets. An operation of the HARQ processor includes operations of transmitting an HARQ packet, receiving HARQ ACK/NACK information in response thereto, and performing retransmission according thereto.
For example, when there is only one HARQ processor, the HARQ processor cannot transmit another packet until it transmits user data and then receives HARQ ACK/NACK information in response thereto. However, when there are several HARQ processors, multiple HARQ processors are enabled while an arbitrary processor waits for an HARQ ACK/NACK, thereby enabling continuous transmission/reception of user data.
A basic operation of the HARQ processor is as follows.
A transmitting-side HARQ processor (an arbitrary HARQ P1 255, HARQ P2 260, HARQ P3 265 and HARQ P4 270) channel-codes a packet received from the multiplexing block 275 before transmission, and stores the channel-coded packet for the future retransmission. Upon receiving ACK information for the transmitted packet from the receiving side, the transmitting-side HARQ processor flushes the stored packet. However, upon receiving NACK information for the transmitted packet, the transmitting-side HARQ processor retransmits the same packet.
A receiving-side HARQ processor (an arbitrary HARQ P1 215, HARQ P2 220, HARQ P3 225 and HARQ P4 230) receives a packet over a physical channel, channel-decodes the received packet, and determines whether there is any error detected in the packet. The error detection can be achieved through a Cyclic Redundancy Check (CRC) calculation.
Upon detecting the presence of an error, the receiving-side HARQ processor stores the packet, and sends an HARQ NACK signal to the transmitting side. Upon receiving a retransmission packet for the packet, the receiving-side HARQ processor soft-combines the stored packet with the retransmitted packet, and then determines again whether there is any error. If it is determined that there is still an error, the receiving-side HARQ processor sends an HARQ NACK signal and repeats the HARQ operation. However, if the error is cleared, the receiving-side HARQ processor sends an HARQ ACK signal to the transmitting side and transfers the user data to the demultiplexing block 210.
In this way, the receiving side requests retransmission of the errored HARQ packet, and soft-combines the retransmitted data with its associated previously received data, making it possible to increase HARQ reception performance. However, it is inefficient to accomplish a very low Block Error Rate (BLER) only with the HARQ operation, for the following two reasons.
1. When an error occurs in the HARQ ACK or NACK signal itself, the HARQ processor cannot detect the occurrence of an error.
2. Since HARQ transmission/retransmission is performed within a relatively short time, the HARQ processor cannot obtain time diversity gain.
For example, when the receiving side falls into a deep fading area for several tens of msec, even though HARQ retransmission is performed, it is hard to finally determine that the HARQ packet has been successfully transmitted. Therefore, an ARQ operation is needed to compensate for the limit of the HARQ operation.
FIG. 3 is a diagram illustrating an operation of retransmitting a packet by applying HARQ and ARQ.
Referring to FIG. 3, an ARQ operation is performed by transmitting-side ARQ layers 361, 362 and 363 and receiving-side ARQ layers 311, 312 and 313.
The transmitting-side ARQ layers 361, 362 and 363, even after transmitting an upper layer packet provided from an upper layer, store the ARQ packet in their own retransmission buffers in preparation for retransmission. The transmitting-side ARQ layers 361, 362 and 363 each configure an ARQ packet(s) corresponding to the amount of data to be transmitted in a transmission period. The transmitting-side ARQ layers can meet the amount of transmission data by making several ARQ packets, or can make one ARQ packet corresponding to the amount of transmission data. If a size of the ARQ packet to be made is not equal to a size of the upper layer packet, the transmitting-side ARQ layers can fragment the upper layer packet to deliver only a part thereof, or can deliver a plurality of upper layer packets. In this case, the transmitting-side ARQ layers configure an ARQ packet by inserting sequence number information, size information and framing information into a desired transmission packet. The transmitting-side ARQ layers deliver the ARQ packet to a lower layer, and store it in their retransmission buffers in preparation for retransmission. Herein, the lower layer is composed of an HARQ layer 370, or a MAC layer, and a physical layer.
The HARQ layer 370 multiplexes the provided ARQ packet to an HARQ packet, and then transmits the HARQ packet to a receiving side over a physical channel, shown at 380.
In the receiving side, a physical layer receives the HARQ packet over a physical channel. A MAC/HARQ layer 320 demultiplexes the received HARQ packet to restore it to an ARQ packet, and then delivers the ARQ packet to the receiving-side ARQ layers 311, 312 and 313. The receiving-side ARQ layers 311, 312 and 313 each are composed of an assembly block, a reception buffer, a retransmission management block, etc. The reception buffer stores an ARQ packet received from the HARQ layer 320 according to its sequence number, and delivers assemblable ARQ packets to the assembly block.
The ARQ retransmission management block checks sequence numbers of ARQ packets stored in the reception buffer, and sending ARQ ACK signals for the received ARQ packets and ARQ NACK signals for missing ARQ packets to the transmitting-side ARQ layers 361, 362 and 363, shown at 341, 342 and 343. The ARQ assembly block reconfigures the original upper layer packet using the ARQ packets depending on framing headers of the ARQ packets provided from the reception buffer, and delivers the upper layer packet to an upper layer.
Upon receiving an ACK signal for the previously transmitted ARQ packet from the receiving-side ARQ layers 311, 312 and 313, the transmitting-side ARQ layers 361, 362 and 363 flush the corresponding ARQ packet from their ARQ retransmission buffers. However, upon receiving a NACK signal, the transmitting-side ARQ layers 361, 362 and 363 schedule retransmission of the corresponding ARQ packet.
As described above, the ARQ layers perform retransmission in units of ARQ packets. The transmitting-side ARQ layers 361, 362 and 363 each attach a sequence number to an ARQ packet before transmission, and the receiving-side ARQ layers 311, 312 and 313 check sequence numbers of the received ARQ packets to determine whether there is any missing ARQ packet.
For example, if a receiving-side ARQ layer has normally received an ARQ packet with a sequence number X and an ARQ packet with a sequence number X+2, but has failed to receive an ARQ packet with a sequence number X+1, the receiving-side ARQ layer sends to a transmitting-side ARQ layer a retransmission request for the ARQ packet with an sequence number X+1. That is, the receiving-side ARQ layer sends a NACK signal to the transmitting-side ARQ layer in response to the non-receipt of the ARQ packet with a sequence number X+1, requesting retransmission of the ARQ packet with X+1.
An HARQ operation performed independently of the ARQ operation is as follows.
The transmitting-side HARQ layer 370 transmits an HARQ packet configured by multiplexing a plurality of ARQ packets. Upon receiving an HARQ NACK from the receiving-side HARQ layer 320 in response to the HARQ packet, the transmitting-side HARQ layer 370 retransmits the HARQ packet. Upon failure to receive an HARQ ACK, the transmitting-side HARQ layer 370 repeats the retransmission operation as many times as the maximum number of retransmissions. If the transmitting-side HARQ layer 370 fails to receive an HARQ ACK even after the transmitting-side HARQ layer 370 repeats the retransmission operation as many times as the maximum number of retransmissions, the transmitting-side HARQ layer 370 abandons transmission of the corresponding HARQ packet, which means transmission failure of the multiplexed ARQ packet.
When the transmission failure caused by the limited number of HARQ retransmissions (Maximum Retransmission Limit) occurs as stated above, fast retransmission based on packet transmission is difficult.
As described above, in the conventional wireless communication system, the HARQ operation and the ARQ operation operate independently of each other. Not only the HARQ ACK/NACK signal exchanged between the transmitting-side HARQ layer and the receiving-side HARQ layer, but also the ARQ ACK/NACK signal exchanged between the transmitting-side ARQ layer and the receiving-side ARQ layer are undesirably transmitted separately by the corresponding processors. As a result, the limited wireless resources cannot be efficiently used by the signals exchanged between the paired layers.
As HARQ and ARQ operate independently of each other, reliable packet retransmission cannot be guaranteed.
Therefore, in the wireless communication system for supporting a high-speed packet service, there is a need for a detection and retransmission scheme of a missing packet(s), in which speed and reliability between the transmitting side and the receiving side is guaranteed. In addition, there is a demand for a packet retransmission scheme capable of efficiently using the limited wireless resources.